At what altitude do adverse effects on night vision typically begin?

Adverse effects on night vision generally begin around 4,000 feet. At this altitude, physiological changes can affect the eye's ability to adapt to low-light conditions, which is crucial for night vision. This is primarily due to the decrease in available oxygen as altitude increases, which can impair the function of rod cells in the retina that are responsible for vision in dim light.

As one ascends to higher altitudes, the effects on night vision can become more pronounced, but the initial impact around 4,000 feet marks a significant threshold where individuals may start to notice difficulties in seeing well at night. The combination of reduced atmospheric pressure and lower oxygen availability contributes to these visual impairments.

It's essential for those operating at higher altitudes, such as pilots or outdoor enthusiasts, to be aware of these effects and take precautions, especially in low-light conditions.